Sunday, March 30, 2008

Why Diesels Are More Torquey

Most would know that diesel engines, even with the same cubic capacity as that of a comparable petrol engine, produce significantly higher torque figures than petrol engines. This article aims to give a simple explanation on why this is the case.

Torque is a function of the force developed atop the piston (due to combustion) and the length through which it acts (the length of the connecting rod). If either of these two increases, torque increases. Let us assume here that the force produced due to combustion is the same for both the same-displacement petrol and diesel engines. Then, the differentiating factor is the length of the connecting rod.

By virtue of their design, diesel engines have higher compression ratios than petrol engines. Compression ratio increases if more parts of charge gets compressed to one part. That is, instead of 6 parts of charge getting compressed to 1 part, if 10 parts get compressed to 1 part, we say that the compression ratio is higher. To compress more parts of charge, the intake of charge should be higher and more specifically the length of the charge mixture needs to higher. Fluids take up the shape of the container that they fill up. So, in diesel engines, to have compress more parts of charge, the stroke of the engine is made to be larger than the bore. This configuration is referred to as the 'under-square' configuration.

Larger stroke means larger length through which the force acts. That is why diesels are inherently more torquier than their petrol counterparts. This is also the reason why they rev lower than petrol engines.


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Chandrasekharan said...

thanks for the info & simple way of explaining... keep it up...

Srivaths said...

Hello Autojunky,

1. Why P220 is not giving good mileage? It is drinking like an elephant!
2. Why cars are giving mileage like bikes - 40 or 50 kmpl?
3. What is the challenge auto companies face to achieve this?

Please guide us.


S R I R A M said...

Srivaths, first up I seriously don't think that you should start worrying about your fuel bills--you're driving a P220 and do justice to it! Anyway, it's a simple rule that as displacement increases, fuel economy decreases. More fuel burnt per unit time will give more power per unit time but sadly, the time for which the fuel is burnt doesn't increase. So, if a Platina engines burns 1 part of petrol per second, for example, then P220 will burn, say, 7 parts of petrol PER SECOND. More fuel, same duration. That's why engine displacement and fuel economy are mostly inversely proportional.

Regarding cars, the weight that the engine has to haul around is much greater in cars than in bikes. This means, more amount of energy is spent to make the extra weight move, even if at the same speed as bikes. That's why no bike-like mileage.

Automobile companies benchmark hybrid cars or diesel cars to improve the efficiency of their engine. I don't think in our generation we can see cars that 'sip' fuel like bikes do.

Hope this helps.

srivath said...

Petrol Head Sir,
Would you agree that all engineers have known this fact for so many years? Then what is the block for them to address this issue. Is Otto cycle that rigid?

If your answer is Hybrid? Then sorry it doesn't fit my pocket or any normal commuter's pocket. Why cannot it be made available to all?
(we had discussed this sometime back)

P220 - P200 is giving mileage of 40 + kmpl. With only 20cc more, P220 is giving 50% of that. What's the reason?

[Rosy picture - Splendour 97 cc is giving 70+ kmpl, XCD 125 is giving 70 + kmpl, here also you have 20 cc difference. ]


S R I R A M said...

Dearest Srivathsan, I think you're trying to be like James May, who argued in the same (pointless) way as you are now. Can you attain the absolute perfection in ONE SINGLE STEP? I cannot do that and apparently the automobile designers cannot do that too. If you think you can do it, I would recommend you quit being the head of Business Marketing in your company, join Maruti and produce an engine that gives 100kmpl to a litre. And which rivals Mercedes-Benz and yet costs a fraction of it.
And I didn't point you into buying a hybrid. I said hybrid technology is the benchmark that petrol engines have to aim to. Hybrid technology will be buyable by the public in the future, if not now. And I think that if you go on asking like this, then I fear you might even ask as to why a Ferrari cannot be bought by a clerk! There's a price that one must accept to pay for getting smarter things in life. It's baseless to refuse to give that price and still demand it.
Regarding the mileage question, it only goes to show how bad you're at the throttle. Some riding lessons might help!

Srivaths said...

Mr Sriram Sridharan,
I am not asking for absolute perfection, all I am asking is Innovation on mileage front. Have you not heard of competitions (even within India)where mechanics squeeze 100 + kmpl out of TVS 50 or other engines?

Again, give me technical explanations and not qualitative explanations.

The last part of my comment had some comparison, you have not taken that into consideration while answering. Please read them once more and answer.